Vanadium oxide microactuators: These are the nanobots you’re looking for

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Nanotechnology has promised us a fantastic world where miniature factories could build devices atom by atom. While semiconductor technology continues to deliver chips with features of ever greater precision, building nano devices that might fertilize this nanoworld — devices that move — have been a little tougher to come by. A recent report in Nano Letters describes a new kind of microactuator made from vanadium oxide. This material has some pretty incredible properties, and it might be able to fill in some of the gaps left by other microactuator technologies — such as piezoelectrics — that come up short.

There was a period before the millennium when collecting beer cans was a legitimate hobby. Many an enterprising youth’s first experiment in chemical physics was freezing a dented can filled with water in an attempt to restore the original shape. Phase changes which significantly alter volume can also occur in some materials without a change in physical state, like going from water to ice. When vanadium dioxide is heated beyond 67 degrees Celsius, it remains in the solid state but undergoes a structural phase transition that expands it in two dimensions while shrinking the third (pictured above). This transition is also accompanied by transformation from an insulator into a metal conductor.

The Berkeley researchers who built the microactuator have utilized this property to create a device that can bend through a distance that is comparable to its length, which is less than the diameter of a human hair. Moreover it can do this at frequencies up to 6 kilohertz. Piezoelectric actuators on comparable scales have been created before, but there are significant shortcomings to the technology. They are extremely fast and fairly strong, but actuating them requires relatively high voltages, and their effective power stroke is small. The cheap sound system in your smoke alarm is often just a piezo disc, which has no trouble being driven at ear-splitting intensities. Though beyond the range of hearing, operation at 40 kilohertz is a walk in the park for a piezoelectrics.

While an enterprising young nanotechnologist could fairly easily get a free sample of piezoelectric material from a vendor to play around with, putting it to work would be a little more difficult. First they would need to cut a relatively hard and brittle material along the preferred direction to the desired shape. Then, once they then have a power supply and controller in hand that can drive it at a couple hundred volts, they still have to find a way to apply it. Typically this requires using an ultrasonic welder to apply leads to the ceramic material or sputtering a conductive surface layer.

Nitinol is a shape-shifting material more familiar to hobbyists that can be actuated at lower voltages. It draws a current which heats the material, causing a change in its grain structure along with a resulting change in shape. Nitinol is great for making things like stents which can be inserted into veins in their compact form, and then expanded to their functional form after cooling just a few degrees. Unfortunately the need to dissipate heat makes them a slow performer when it comes to building practical actuators. Small devices need to operate fast, particularly if they are ever going to be used to build something large enough to see or interact with.

Vanadium oxide actuators (video above) need only a small current to drive them, and they can even be contracted with a pulse of laser light. They therefore seem to offer the best of both worlds, and may soon find application in practical devices.

As these devices are constructed now at the microscale, rather than the nanoscale, what might be the immediate applications? The researchers have suggested that the actuators could be used as tiny pumps for drug delivery, or as mechanical micro-muscles. Pound for pound, they could deliver a force 1000 times greater than our muscle. Other applications may use the device operated as a torsion device; a more complicated geometry that the group is now working on. With devices like these we are given the next set of hands, a little smaller, which may build the hands that hand us the atoms.

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